A ball table having multiple ball transfer units arranged on a fixed disk or other support part is used to correct the transport position of a transported material on its transport path or to change the transport direction to the perpendicular direction. The ball transfer unit assembled in such a ball table has a main body having a seat surface recessed in a semispherical shape, multiple small balls rollingly in contact with the seat surface of the main body, a large ball rollingly in contact with the multiple small balls, and a cover installed on the main body to hold the large ball and to hold small balls between the large ball and the seat surface of the main body. In the aforementioned ball transfer unit, when the large ball rolls and the small balls having contact with the large ball and the seat surface of the main body roll between them along with movement of the transported material carried on the large ball, the static friction resistance between the transported material and the large ball can be reduced to a very low level.
Consequently, the transported material can be easily displaced with respect to external force in any direction on the transport surface of the transported material on the ball table. The transport position of the transported material can be corrected very easily on its transport path. For example, U.S. Pat. No. 2,641,187 disclosed the following technology. In this case, the side end acting as the positioning reference for automobile window glass with a large weight or other material transported on a ball table is pressed by an actuator against a positioning reference block fixed with respect to the ball table. In this way, the transport position of the transported material is corrected.
Japanese Kokai Patent Application No. Hei 7[1995]-164078 discloses a ball transfer unit, which is made of a plastic that is softer than metal to prevent scratches from forming on the surface of metal plates transported thereon, and is also self-lubricating to prevent lubricant from adhering to the metal plates.
For a production line used to form a circuit on a semiconductor wafer or a production line for a flat panel display, when transporting a semiconductor wafer or glass substrate, it is necessary to determine their positions in each specific step. The ball table can be used for such a positioning operation. When transporting a semiconductor wafer or a glass substrate for a flat panel display, it is necessary to prevent damage to the surface caused by friction and the attachment of foreign matter. Even if foreign matter is attached, it is necessary to make sure that it can be easily removed by means of washing.
In consideration of the aforementioned point of view, the conventional technologies disclosed in U.S. Pat. No. 2,641,187 and Japanese Kokai Patent Application No. Hei 7[1995]-164078 have problems because these technologies will cause damage to the surface of the transported material, or will attach foreign matter that will cause defects in the transported material, or will attach foreign matter that cannot be removed by means of washing in a later step. For example, when the main body or the large ball is made of stainless steel or other metal, the metal powder generated as a result of abrasion will cause damage to the surface of the transported material or will be attached as foreign matter that cannot be removed by means of washing in a later step on the surface of the transported material. In the ball transfer unit made of polyurethane or polyacetal used in patent reference 2, the resin powder generated as a result of abrasion is fixed as clear traces on the surface of the transported material, which makes the washing operation in a later step very difficult.